In open drain transmitters, such as those used in high definition multi-media interface (HDMI) operation, a receiver provides a common mode bias voltage across receiver terminating resistors to an open drain of a transmitter. FIG. 1 shows an example of a typical system 100 with an open drain transmitter/receiver configuration. A transmitter 110 comprises a differential pair of open drain transistors 111 which transmit differential data (D) over lines TX− and TX+ to a receiver 120 over the channel 130 having an impedance Z0. The transistors 111 draw current from a voltage source (AVcc) 121 of the receiver 120 and a voltage drop across terminating resistors (RT) 123 and 122 at the receiver is used to determine the transmitted data. A current source 112 in the transmitter 110 is used to determine the current drawn from the receiver (this current may have a set limit in accordance with a transmitter protocol). Typically, the current is fed back to the receiver via a ground path 113 coupled between the transmitter 110 and receiver 120.
Reference is now made to FIG. 2. It is known in the art to harvest this current to provide power to the transmitter. Using power harvested from the current would negate the need for providing an additional power source for the transmitter. In other words, an independent power supply need not be provided in the transmitter itself. However, because a pre-amplifier circuit is conventionally used to switch the data signal passed by the transmitter, the harvested current requires a voltage boost in order to maintain a suitable voltage difference for operation of the preamplifier circuit. Level shifters within the pre-amplifier introduce speed limits on the transmitter output. Power efficiency with such power harvesting circuitry is also a concern.
FIG. 2 shows an open drain differential pair of transistors NL0 and NL1 (for example, n-channel MOSFETs). The gate terminals of transistors NL0 and NL1 receive differential data output from a pre-amplifier circuit 220 and the drain terminals of the transistors NL0 and NL1 are connected to pads TX− and TX+(coupled through channel 130 to the receiver 120 as shown in FIG. 1). Data is input to the pre-amplifier circuit 220 from a serializer circuit 210. A drive current Idr through the differential transistors NL0 and NL1 is determined by a current sensor/control block 241 that receives a bias signal. The current sensor/control block 241 forms part of a voltage recovery (power harvesting) circuit 240 which recovers the power in the drive current through use of a voltage regulator 243 to provide a supply voltage Vdd to the serializer 210, preamplifier 220 and perhaps other circuitry.
In order to effectively switch the transistors NL0 and NL1, the pre-amplifier 220 may require a supply voltage Vdd that is higher than a voltage regulated solely from the drive current. In such a case, a voltage booster 242 is accordingly provided in circuit 240 to boost the voltage prior to regulation and the output of voltage Vdd to the pre-amplifier 220. For example, the drive current alone may be sufficient to generate logic level power supply voltage. However, other circuitry, such as phase-lock-loop (PLL) circuitry, may need a power supply voltage that is higher than logic level. Operation of the transmitter in a fully power harvested implementation is not typically possible due to the need for multiple different power supply voltages. If the multiple voltages are provided by a booster circuit, such as through the use of booster 242, there is a loss of power efficiency.
There is accordingly a need in the art for multiple supply voltage power harvesting in open drain transmitter circuits. Such a power harvesting solution would desirably provide at least two different power supply levels including a logic level power supply and a higher power supply. Furthermore, such a solution would not require the use of a voltage booster circuit.